The present invention relates to an automatic frequency control apparatus capable of improving transmission quality in the digital modulation/demodulation system employed in satellite communications, mobile communications, and mobile satellite communications. More specifically this invention relates to an automatic frequency control apparatus capable of improving receiver performance by periodically inserting a known signal into an outgoing signal and removing a frequency deviation generated in the known signal during transaction between the transmitter and the receiver.
In recent years, in the fields of satellite communications, mobile communications, and mobile satellite communications, research is actively being performed on digital modulation/demodulation. Especially, in the mobile communications, an incoming signal is received which is generally subjected to severe fading, so that now feasibility of various types of demodulation system enabling stable operations even in the fading environment are being examined. Under such circumstances, a technology in which a known signal to be utilized for measuring a distortion due to fading in a channel is periodically inserted into the outgoing signal, the distortion due to fading is estimated and compensated from the known signal has been gathering hot attentions as a technology enabling coherent detection even in the fading environment. When performing quasi-coherent detection using such a system, it is required to estimate and compensate the distortion due to fading with high precision, and further the frequency deviation between the outgoing carrier wave and a reference signal for performing quasi-coherent detection in a receiver should be small.
However, when performing coherent detection in the fading environment, if stability and precision of a frequency of an oscillator in a transmitter/receiver are not sufficient, it is impossible to estimate and compensate the distortion due to fading with high precision unless some other specific processing is disadvantageously executed.
In the field of mobile communications, signal transaction is carried out between two mobile stations or between a stationary station and a mobile station, so that, when two stations are relatively moving, a frequency deviation occurs in the transmitted electric wave due to the Doppler effect. Even if the precision of the oscillator in the transmitter or the receiver is high, a frequency deviation disadvantageously occurs between the outgoing carrier wave and a reference signal for performing quasi-coherent detection in a receiver.
As a technology for solving the problems as described above, there is, for instance, a technology described in xe2x80x9cFrequency Offset Compensation Method for QAM in Land Mobile Radio Communicationsxe2x80x9d (Kato, Sasaoka, Collection of reports, Institute of Electronics, Information and Communication Engineers (B-II), J74-B-II, No.9, pp493-496 (1991-9)).
FIG. 19 is a view showing configuration of a conventional type of automatic frequency control apparatus described in the above document. In FIG. 19, designated at the reference numeral 80 is a known-signal distortion detecting section, designated at 89 is a inter-known-signal phase difference estimating section, designated at 890 is a inter-known-signal phase difference computing section, designated at 891 is a averaging section, and designated at 85 is a inter-one-symbol phase difference computing section.
Operations of the conventional type of automatic frequency control apparatus as described above are explained below.
FIG. 20 is a view showing a format of an incoming signal when a one-symbol known signal is periodically inserted in the signal. For instance, a transmitter transmits a signal formatted by periodically inserting a one-symbol known signal (herein, a known pilot signal) into a (NFxe2x88x921) symbol data signal as shown in FIG. 20. It is assumed herein that the one-symbol pilot signal is inserted at a time t=kNFTS. Herein k indicates a natural number, NF indicates an interval at which the pilot signal is to be inserted (insertion interval), and TS indicates a symbol duration.
When such a signal is received by the known-signal distortion detecting section 80, the inter-known-signal phase difference computing section 890 in the inter-known-signal phase difference estimating section 89 computes a phase deviation between the pilot signals inserted at an interval of NF. Further, the averaging section 891 calculates the average of the phase deviations outputted from the inter-known-signal phase difference computing section 890 and outputs the average  less than xcex8(kNF) greater than  of the phase deviations xcex8(kNF).
Then the inter-one-symbol phase difference computing section 85 computes a phase rotation xcex8S in one-symbol as expressed, for instance, by the equation (1) from the average of phase deviations.                               θ          S                =                              ⟨                          θ              ⁢                              xe2x80x83                            ⁢                              (                                  kN                  F                                )                                      ⟩                                N            F                                              (        1        )            
Then the inter-one-symbol phase difference computing section 85 executes integration processing by means of iterative addition of one-symbol cycles as expressed, for instance, by the equation (2) by using the computed phase rotation xcex8S.
xcex8(kNF+i)=xcex8(kNF+ixe2x88x921)+xcex8Sxe2x80x83xe2x80x83(2)
Finally the inter-one-symbol phase difference computing section 85 rotates the phase of the incoming signal to remove the frequency deviation. Namely, the inter-one-symbol phase difference computing section 85 rotates the phase for each one-symbol, as expressed by the equation (3), for a digital baseband signal r(kNF+i) in the I channel and Q channel to remove the frequency deviation.
rR(kNF+i)=r(kNF+i)exp[xe2x88x92jxcex8(kNF+i)], 0xe2x89xa6ixe2x89xa6NFxe2x88x921xe2x80x83xe2x80x83(3)
Assuming that the modulating symbol rate is RS (symbol/s), a frequency deviation detection range xe2x88x92fDET[Hz] to fDET[Hz] in the inter-known-signal phase difference estimating section 89 xe2x88x92fDET[Hz] to fDET[Hz] is as expressed by equation (4):                               f          DET                =                              R            S                                2            ⁢                          N              F                                                          (        4        )            
As understood from the equation (4), by reducing the insertion interval NF the range of detectable frequency deviation can be made wider. However, when it is considered that a function of the inter-known-signal phase difference estimating section 89 is equivalent to that of a filter, when the range of frequency deviation detection is made wider by reducing the insertion interval NF, the same effect as that provided when a frequency band of a filter is equivalently extended is provided. Namely, an estimation error for a frequency deviation due to noises or the like becomes larger, and precision in estimating a frequency deviation is degraded. Accordingly, to improve the precision in estimating a frequency deviation, it is better that the insertion interval NF is large.
In the automatic frequency control apparatus as described in the above mentioned document, however, the amount of phase rotation due to a frequency deviation is computed by using only the known signals inserted at an insertion interval of NF, so that it is necessary to increase the insertion interval NF in order to improve the precision in estimation of a frequency deviation. Due to this fact, the detection range of frequency deviation becomes disadvantageously narrower. On the other hand, if the insertion interval NF is decreased in order to widen the detection range of frequency deviation, the frame efficiency becomes lower, and there occurs a new problem that the precision in estimation of the frequency deviation is degraded.
Further, in the conventional type of automatic frequency control apparatus, a distortion of a channel is detected by using only a one-symbol known signal, so that a period of time required for high precision in estimating a frequency becomes longer in an environment with a low C/N (carrier to noise power ratio) and the tracking performance to time-dependent fluctuation in a frequency deviation caused by the Doppler effect or the like becomes disadvantageously lower.
Further, when a direct wave and a multi-path wave coexist like in the fading environment, a frequency deviation of the direct wave can not be estimated with high precision because of effects by the multi-path wave, and frequency control over the direct wave becomes disadvantageously difficult.
The present invention was made under the circumstances as described above, and it is an object of the present invention to provide an automatic frequency control apparatus which enables simultaneous realization of a wide detection range for a frequency deviation and high precision in estimation of a frequency deviation as well as estimation of a frequency deviation within a short period of time even in an environment with a low C/N ratio. It is also an object of the present invention to provide an automatic frequency control apparatus which allows estimation of a frequency deviation in a direct wave with high precision and frequency control over the direct wave even in a situation where a multi-path wave and a direct wave coexist like in the fading environment.
In the present invention, a frequency deviation in one symbol is computed according to the frequency deviation in the first period estimated by the first phase difference estimating unit and the frequency deviation in the second period estimated by the second phase difference estimating unit. Therefore, a wide detection range for a frequency deviation in one of the phase difference estimating units and high precision in estimation of a frequency deviation in the other phase difference estimating unit can simultaneously be realized. In addition, with the present invention, a frequency deviation of a direct wave can be estimated with higher precision than that of the conventional system in a situation where a multi-path wave and a direct wave coexist like in the Rician fading environment, and further accurate frequency control can be provided to a direct wave. The phase difference herein includes a meaning of information for a phase difference represented by a vector other than an ordinary phase difference xcex8.
In the present invention, the distortion averaging unit executes the average processing, therefore an estimation error for phase fluctuation due to fading as well as for a frequency deviation affected by noise or the like can be reduced, and at the same time a frequency deviation between a transmitter and a receiver and frequency fluctuation caused due to the Doppler effect can be estimated with high precision.
In the present invention, the first phase difference estimating unit located in the previous stage of the second phase difference estimating unit estimates a frequency deviation in N1, and after the frequency deviation is removed according to a result of the estimation, the second phase difference estimating unit further estimates a frequency deviation in N2, and the results of estimation of frequency deviations in both the phase difference estimating units are synthesized. Therefore, a wide detection range for a frequency deviation in the first phase difference estimating unit and high precision in estimation of a frequency deviation in the second phase difference estimating unit can simultaneously be realized. In addition, with the present invention, a frequency deviation of a direct wave can be estimated with higher precision than that of the conventional system in a situation where a multi-path wave and a direct wave coexist like in the Rician fading environment, and further accurate frequency control can be provided to a direct wave. By performing operation with only phases in the first phase difference estimating unit and the second phase difference estimating unit, removal processing of a frequency deviation can be realized by an apparatus with simple a configuration.
In the present invention, the first phase difference estimating unit located in the previous stage of the second phase difference estimating unit estimates a frequency deviation between adjacent known signals, and after the frequency deviation is removed according to a result of the estimation, further, the second phase difference estimating unit estimates a frequency deviation in N2, and the results of estimation of the frequency deviation in both the phase difference estimating units are synthesized. Therefore, a wide detection range for a frequency deviation in the first phase difference estimating unit and high precision in estimation of a frequency deviation in the second phase difference estimating unit can simultaneously be realized. In addition, with the present invention, a frequency deviation of a direct wave can be estimated with higher precision even in a situation where a multi-path wave and a direct wave coexist like in the Rician fading environment, therefore accurate frequency control can be provided to a direct wave.
In the present invention, the first phase difference estimating unit estimates a frequency deviation in N1 and the second phase difference estimating unit estimates a frequency deviation in N2 regardless of the order thereof, and a frequency deviation in one symbol is computed according to both of these frequency deviations. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit and high precision in estimation of a frequency deviation in the second phase difference estimating unit can simultaneously be realized.
In the present invention, the phase difference selecting unit selects the first phase difference estimated by the first phase difference estimating unit or the phase difference estimated by the second phase difference estimating unit. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit or high precision in estimation of a frequency deviation in the second phase difference estimating unit can be selected.
In the present invention, the first inter-symbol phase difference computing unit and the second inter-symbol phase difference computing unit are located behind the first phase difference estimating unit and second phase difference estimating unit respectively, each of them compute a phase difference in one symbol, and then the phase difference selecting unit selects either one of the outputs from the inter-symbol phase difference computing units. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit or high precision in estimation of a frequency deviation in the second phase difference estimating unit can be selected.
In the present invention, the first phase difference estimating unit estimates a frequency deviation between adjacent known signals and the second phase difference estimating unit estimates a frequency deviation in N1 regardless of the order thereof, and a frequency deviation in one symbol is computed according to both of the frequency deviations. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit and high precision in estimation of a frequency deviation in the second phase difference estimating unit can simultaneously be realized.
In the present invention, the phase difference selecting unit selects the first phase difference estimated by the first phase difference estimating unit or the phase difference estimated by the second phase difference estimating unit. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit or high precision in estimation of a frequency deviation in the second phase difference estimating unit can be selected.
In the present invention, the first inter-symbol phase difference computing unit and the second inter-symbol phase difference computing unit are located behind the first phase difference estimating unit and second phase difference estimating unit respectively, each of them compute a phase difference in one symbol, and then the phase difference selecting unit selects either one of the outputs from the inter-symbol phase difference computing units. Therefore, for example, a wide detection range for a frequency deviation in the first phase difference estimating unit or high precision in estimation of a frequency deviation in the second phase difference estimating unit can be selected.
In the present invention, average processing of phase differences in a burst is performed to a burst signal in TDMA, further average processing of phase differences over bursts is performed, and a value of a forgetting factor as a parameter in the weighted average is appropriately selected. Therefore, an estimation error for phase fluctuation due to fading as well as for a frequency deviation affected by noises or the like can be reduced.
In the present invention, a phase difference in one symbol is computed, and average processing is executed to phase differences over symbols. Therefore, an estimation error for phase fluctuation due to fading as well as for a frequency deviation affected by noises or the like can further be reduced.
In the present invention, as only the known signal is used for configuration thereof, a frequency deviation can be removed with simple processing, with which it is possible to realize improvement of receiver performance. In addition, with the present invention, as desired precision of estimation of a frequency can be obtained in a comparatively short period of time even in an environment with a low C/N, the tracking performance to time-dependent fluctuation in a frequency deviation caused by the Doppler effect or the like becomes excellent. With the present invention, a frequency deviation of a direct wave can be estimated with higher precision even in a situation where a multi-path wave and a direct wave coexist like in the Rician fading environment, therefore high-precision frequency control can be provided to a direct wave.
In the present invention, a phase deviation is removed before waveform shaping is executed to a digital baseband signal by means of filtering processing. Therefore, even when a frequency deviation is larger as compared to a cutoff frequency of a LPF, the frequency deviation can be removed without reduction of a portion of signal power.
In the present invention, the oscillator control unit controls a voltage controlled oscillator according to a frequency deviation detected by the frequency deviation estimating unit. Therefore, even when a frequency deviation is larger as compared to a cutoff frequency of a LPF, the frequency deviation can be removed without reduction of a portion of signal power.
In the present invention, the oscillator control unit or phase rotating unit is switched according to a frequency deviation detected by the frequency deviation estimating unit. Therefore, even when a frequency deviation is large at the time of capturing a signal in an initial stage, the frequency deviation can be removed without reduction of a portion of signal power by means of a cutoff frequency of a LPF. In addition, with the present invention, switching control of a frequency deviation is executed according to information for frequency deviation from the phase difference computing unit, therefore an optimal removal of frequency deviation according to the information for the frequency deviation can be made.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.